1. Field of the Invention
The invention relates in general to measurements by means of which a radio apparatus attempts to find out the quantity and quality of radio-frequency oscillation in its operating environment. In particular the invention relates to measurements directed to frequencies other than that used for transmission and reception by a radio apparatus in a code division system during the measurements.
2. Brief Description of Related Developments
A communications link between a cellular radio system base station and terminal requires that the terminal transmits at a certain first frequency and receives at a certain second frequency which may be the same as said first frequency if the transmission and reception are otherwise separated by means of a duplexing method. Optimal link quality requires that the terminal selects a base station with a strong signal, and a frequency with as little noise and interference as possible.
Cellular radio systems applying code division multiple access (CDMA) have so far been mainly so-called single frequency networks (SFN) in which the whole system uses only one frequency band. This kind of an arrangement is used e.g. in the North-American IS-95 (Interim Standard 95) system. Frequency ranges used by other types of cellular radio systems are usually divided into several parallel frequency bands which can be called carrier wave frequencies or just frequencies in short. Proposals for future cellular radio systems have also put forward systems based on code division multiple access which have several frequency bands e.g. to separate hierarchically arranged cells from each other. This requires that a system be developed with which a receiver in such a system can carry out measurements not only at the frequency used, but also at other frequencies in order to find out how much there is other traffic and noise at the other frequencies.
A measurement means that the receiver in a terminal is tuned to the frequency measured and the required observations are made concerning the quantity and quality of the signal received, such as the mean power level at the frequency measured and its temporal distribution, for example. The receiver is tuned by changing the mixing frequency brought to a mixer in the receiver. When the receiver is tuned to the frequency measured it naturally cannot simultaneously receive a signal at the communications frequency used. So, the measurement of parallel frequencies cannot be carried out coincidentally with the reception of the desired signal at the communications frequency used.
It is known from patent document U.S. Pat. No. 5,101,501 some arrangements for realising a handover in a CDMA-type cellular radio system, applicable in single frequency networks. In an arrangement discussed by said document in conjunction with the description of the prior art, measurements are carried out not by the terminals but by the base stations. This arrangement requires that each base station has at all times free demodulating units that can be instructed to receive and measure a transmission by a terminal the signal of which in a neighbouring cell is weakening, thus indicating that the terminal is moving toward the inter-cell boundary. The transmission from the terminal is received, if necessary, by both base stations and the total responsibility for the connection is handed over to the new base station only when the terminal has clearly crossed the inter-cell boundary. In this method the base station equipment is continuously under-utilised since there has to be so many demodulating units that some of them are always free. Furthermore, the inter-base station signalling needed for the coordination of connections received via two routes puts a heavy load on the base station systems and their mutual communications connections.
Said patent document also discloses an improved arrangement in which the terminals measure a so-called pilot channel transmitted by the base stations. The implementation of the measurement is in practice rather simple as all base stations transmit at the same frequency. A terminal need not tune its receiver separately to other frequencies for measurement. It is obvious that this arrangement does not work if the different base stations transmit at different frequencies.
A known proposal for the arrangement of reception and measurement timing in a multifrequency CDMA system is a so-called compressed mode in which base station transmissions comprise successive frames in the normal manner but in which certain frames are transmitted at double speed so that half of the time reserved for such frames is left free for the measurements.
The proposed compressed mode involves several problems. The terminals have to be capable of receiving at a speed two times higher than the normal reception speed, which makes their structure and operation more complex. Compressing the frames means compromises in the implementation of control functions such as transmission power adjustment, which degrades the performance of the whole system. Pauses in the reception result in more complex continuous-operation algorithms, such as the measurement of propagation delay and channel estimation. Operation control for the compressed mode increases the signalling between the base stations and terminals.
In addition, the compressed mode is bound to cause deterioration of the bit error ratio, which is reflected in a decrease in the system performance.
An object of the present invention is to provide a method and equipment with which a terminal in a multifrequency, continuous-transmission radio communications system can make measurements not only at the operating frequency but also at other frequencies without the disadvantages characteristic of the prior art.
The objects of the invention are achieved by using in the terminal a diversity receiver and by switching one receiver branch at times to measurement use so that during measurement, the other branches of the receiver are receiving normally.